The present invention relates to nitric oxide donors containing at leas one sulfhydryl group or a group capable of being converted in-vivo to a sulfhydryl group, and at least one nitric oxide donor group. The novel compounds are effective substitutes for existing tolerance-inducing organic or inorganic nitric oxide donors.
For over a century, the nitric oxide (NO) donor nitroglycerin (GTN) has been the mainstay in the treatment of angina and heart diseases. However, the existing mechanisms proposing the mediation of GTN action by free NO, intracellular or extracellular S-nitrosothiol formation and subsequent activation of guanylyl cyclase (GC), as well as those describing GTN tolerance, have become increasingly controversial. The phenomenon of tolerance to GTN, however, is of special clinical importance. In fact, early tolerance to the anti-anginal effects of the drug is the major drawback of nitrate therapy, especially during acute myocardial infarction. This is particularly important since alternative non-tolerance inducing agents have not yet been developed to successfully replace therapy with GTN.
Based on accumulating evidence, Applicant hypothesized that GTN may directly interact with SH-group/s located on its target enzyme (GC) resulting in its S-nitrosylation and activation. However, subsequent auto-oxidation (disulfide formation) of these SH-groups renders the enzyme inert towards further action with GTN, resulting in tolerance development.
Additionally, evidence has recently been provided to support an involvement of the superoxide anion in the mechanism/s underlying GTN tolerance and cross-tolerance. According to these reports, increased production of superoxide anion was found to accompany tolerance development to GTN in vascular tissue. Treatment with superoxide dismutase (SOD) significantly enhanced relaxation of control and tolerant vascular tissue to GTN and other exogenous and eudogenous vasodilators.
While the precise mechanism for the vasorelaxant effect of GTN is unknown, a consensus exists regarding the primary involvement of cGMP in mediating the nitrate-induced relaxation. However, the roles of sulfhydryl groups [reduced glutathione (GSH) and cysteine (Cys)] and of various enzymes in the bioconversion of GTN and subsequent activation of guanylyl cyclase (GC) leading to relaxation have become increasingly controversial. Cysteine was found to be the specific sulfhydryl required for activation of soluble coronary arterial GC and to be the only one of several sulfhydryls to react non-enzymatically with GTN at physiologic pH resulting in formation of S-nitrosocysteine. Since S-nitrosothiols were shown to be potent activators of GC, S-nitrosocysteine/thiols were proposed as the intracellular mediators of organic nitrate-induced vasorelaxation. Additionally, N-acetylcysteine (NAC, an immediate donor of Cys thereby increasing GSH) was reported to potentiate GTN activity in vitro and in vivo. The enhanced reaction of thiols with GTN in plasma and blood versus buffer suggested that activation of GC by GTN may be mediated via extracellular formation of S-nitrosothiol/s. In either case (intra- or extracellular S-nitrosothiol formation), this association between sulfhydryls and GTN activity has long been recognize as evidence for the xe2x80x9cthiol depletion hypothesisxe2x80x9d. However, recent studies by the Applicant and those of Boesgard et al. revealed a dissociation between tissue thiol content (measured as Cys and GSH) and nitrate tolerance in vivo.
In vitro inhibitory studies provide indirect support for the involvement of enzymes in GTN bioactvation [glutathione S-transferase (GST) and cytochrome P450 (P-450)]. However, in view of several other reports suggesting the lack of any significant role of GST and P-450 in GTN bioactvation, the reduced bioactvation of GTN is unlikely to be the main factor underlying nitrate tolerance in vivo. In fact, reduced cGMP production was also shown to follow exposure of vascular preparation to direct NO-donors, for which no definitive metabolic pathway has been reported.
Furthermore, Applicant has recently presents in vivo evidence excluding the involvement of any particular metabolic pathway since reduced cGMP was also shown to follow treatment with S-alkylating agents in the absence of GTN.
Heart disease is the leading cause of death in Western society and is rapidly approaching this leading position worldwide. Ischemic heart disease is the most common heart disease. For over a century, nitroglycerin and other organic nitrates have been used for the treatment of various types of myocardial ischemia, including acute myocardial infarction (AMI) and as adjuncts in the treatment of other heart disease (congestive heart failure and instant hypertension). Chronic prophylaxis and acute treatment am necessary to prevent complications of ischemic heart disease with potential fatal outcomes (xcx9c25% death for AMI). Tolerance to the anti-ischemic effect of these drugs is, by far, the most serious drawback of therapy with currently available organic nitrates. The compounds proposed in this application constitute a novel approach to overcome tolerance.
The present invention provides a compound containing at least one sulfhydryl group and at least one NO donor group, wherein said compound contains one or more protected sulfhydryl groups linked to at least one aromatic ring or a heteroaromatic ring with a nitrogen in the ring structure, which ring is substituted by one or more substituents bearing at least one terminal xe2x80x94ONO2 group.
In one embodiment, the protected sulfhydryl group is an acetylated sulfhydryl group.
In one embodiment, the compound is: 
The present invention further provides a pharmaceutical composition comprising i) as an active ingredient at least one compound containing at least one sulfhydryl group and at least one NO donor group, wherein said compound contains one or more protected sulfhydryl groups linked to at least one aromatic ring or a heteroaromatic ring with a nitrogen in the ring structure, which ring is substituted by one or more substituents bearing at least one terminal xe2x80x94ONO2 group; and ii) a pharmaceutically acceptable carrier.
In one embodiment, the protected sulfhydryl group is an acetylated sulfhydryl group.
In one embodiment, the active ingredient is: 